Carbon dioxide blasting systems are well known, and along with various associated component parts, are shown in U.S. Pat. Nos. 4,744,181, 4,843,770, 4,947,592, 5,018,667, 5,050,805, 5,071,289, 5,109,636, 5,188,151, 5,203,794, 5,249,426, 5,288,028, 5,301,509, 5,473,903, 5,520,572, 5,571,335, 5,660,580, 5,795,214, 6,024,304, 6,042,458, 6,346,035, 6,447,377, 6,695,679, 6,695,685, and 6,824,450, all of which are incorporated herein by reference. Additionally, U.S. patent application Ser. No. 11/344,583, filed Jan. 31, 2006, for PARTICLE BLAST CLEANING APPARATUS WITH PRESSURIZED CONTAINER, U.S. patent application Ser. No. 11/853,194, filed Sep. 11, 2007, for PARTICLE BLAST SYSTEM WITH SYNCHRONIZED FEEDER AND PARTICLE GENERATOR, U.S. patent application Ser. No. 12/121,356, filed May 15, 2008, for PARTICLE BLASTING METHOD AND APPARATUS THEREFOR, U.S. patent application Ser. No. 12/348,645, filed Jan. 5, 2009, for BLAST NOZZLE WITH BLAST MEDIA FRAGMENTER, U.S. Patent Provisional Application Ser. No. 61/394,688 filed Oct. 19, 2010, for METHOD AND APPARATUS FOR FORMING CARBON DIOXIDE PARTICLES INTO BLOCKS, and U.S. Patent Provisional Application Ser. No. 61/487,837 filed May 19, 2011, for METHOD AND APPARATUS FOR FORMING CARBON DIOXIDE PARTICLES, are hereby incorporated by reference.
In a particle blast system, typically, particles, also known as blast media, are ejected by a particle acceleration device, generally referred to as a blast nozzle, and directed toward a workpiece or other target (also referred to herein as an article). Particles may be introduced into a transport gas flow through a feeder, such as is disclosed in U.S. Pat. No. 6,726,549, which is incorporated herein by reference, and transported by the transport gas, entrained therein, from the feeder to the blast nozzle through a single hose (known as a one hose system). It is also known to introduce particles into the high pressure gas at the blast nozzle, the blast nozzle being configured to combine the particle flow arriving entrained in a low volume gas flow through a first hose with high pressure gas arriving in a second hose and eject the entrained flow therefrom (known as a two hose system).
Various sizes are known for carbon dioxide blast media, such as pellets and granules, the selection of which is made in dependence on the blasting needs. Pellets may be formed by extruding carbon dioxide snow through a die plate. Pellet diameters come in various sizes, for example ranging from 3 mm to 12 mm. Granules may be formed by any suitable process, such as by use of the apparatus for generating carbon dioxide granules from a block, referred to as a shaver, as is disclosed in U.S. Pat. No. 5,520,572, which is incorporated herein by reference, in which a working edge, such as a knife edge, is urged against and moved across a block of carbon dioxide. As shown in the '572 patent, the granules so generated are fed directly into the low volume gas flow, such as by Venturi induction as shown in FIG. 1 of the '572 patent, transported by the first hose to the blast nozzle 102 ('572, FIG. 6) where it is combined with the high pressure gas and directed toward a workpiece.
Unwanted sublimation of the carbon dioxide blast media occurs prior to the media reaching the workpiece whenever the environmental conditions allow. Sublimation of granules can be a significant problem, due at least in part to the very small mass of each individual granule relative to its volume and surface area. For example, the '572 patent teaches to deliver the granules, generated by shaving a dry ice block, directly into the first hose of the two hose system with substantially no storage of the granules to be transported to be combined with the high pressure gas.
Until the present invention, due to sublimation, systems utilizing granules were limited to low flow apparatuses. Double hose and single hose granule systems were known, but high flow systems were not. Two hose systems using granular blast media were typically limited to low flow, with a maximum hose (for transporting granules) internal diameter of ¾″ and maximum length of 50 feet. Previously, persons of greater than ordinary skill in the art designed such systems to avoid high volume gas flow based on the conclusion that the sublimation rate of granules was proportional to the volume of the flow of gas in which the granules were entrained, leading to prior art systems maintaining low flow through small hose diameters for hoses. Attempts at using large diameter hoses in single hose systems resulted in systems with sublimation rates that required granular media flow rates of 10 to 20 lbs per minute just to equal the results of the two hose systems delivering 5 lbs per minute. Such result reinforced the continued use of smaller hose diameters.
The present inventors have overcome the problems unsolved by such persons of more than ordinary skill in the art, and successfully configured a single hose granular blast media system capable of delivering high flow, based on their determination that the sublimation problem was not the result of the volume of the gas flow that entrained the granules, but rather was the result of the velocity of the gas flow in which the particles were entrained. The inventors have determined that it is the difference between the speed of the gas flow and the speed of the granules that results in sublimation: The greater the difference the greater the sublimation. Applying the inventors' discovery to the prior art attempts at single hose granular blast media systems, it is now to be understood that the increase in sublimation that accompanied use of a larger cross sectional area hose (i.e., the larger diameter hose), which was misinterpreted by those of more than ordinary skill in the art as resulting from increased flow volume, was the result of increased gas velocity resulting from use of nozzles which that increased the gas speed in the hose (instead of decreasing gas speed which, with increased cross sectional area, would be expected to decrease speed). However, the inventors' present invention overcomes the misunderstandings, misinterpretations and shortcomings of the prior art by providing a single hose granular blast media system with high flow configured to maintain the speed differential between the transport gas and the entrained granules low enough to keep sublimation rates low enough to be functionally acceptable.
Although the present invention will be described herein in connection with a particle feeder for use with carbon dioxide blasting, it will be understood that the present invention is not limited in use or application to carbon dioxide blasting. The teachings of the present invention may be used in applications using particles of any sublimeable and/or cryogenic material.
Reference will now be made in detail to an embodiment of the invention, an example of which is illustrated in the accompanying drawings.